Reconditioning acidic and artificially aged cellulose with alkaline nanoparticles: an NMR diffusometry study

Abstract

The prominent degradation mechanism of cellulose is the acid-catalyzed hydrolysis of glycosidic bonds, which results in the decrease of the degree of polymerization (DP) and, macroscopically, in the dramatic decay of the mechanical resistance of cellulose-based materials. Alkaline nanoparticles in organic solvents have been recently proposed for the deacidification of cellulose-based artworks. Their effectiveness has been demonstrated in previous studies, by pH and DP measurements, colorimetric and thermal analyses. Herein, the changes in the cellulosic network following an acidification bath and a consequent deacidification treatment using Ca(OH) 2 nanoparticles, have been investigated by NMR selfdiffusion dynamics of water and related to the changes of samples' DPs. The deacidification treatment modifies intra-and inter-chain interactions, leading to a buffered cellulose network configuration similar to that characterizing the untreated reference sample in terms of diffusive parameters and confining environment. Such results are plausibly due to a rearrangement in connectivity of the cellulosic network, even though with a different physical fingerprint with respect to the reference sample. The analysis of tortuosity of the cellulosic network in acidic and deacidified samples confirms this conclusion, further corroborating the idea that calcium hydroxide nanoparticles are an effective tool to hamper the degradation of cellulose induced by acids and aging in strong environmental conditions, even from the standpoint of cellulose network arrangement.

Detection of acidic paper recovery after alkaline nanoparticle treatment by 2D NMR relaxometry

“…Such data return the following confining sizes: d = 2.3 ± 0.1 μm for the D sample, d = 2.2 ± 0.2 μm for the D1 sample, and d = 2.0 ± 0.2 μm for the D2 sample, in full consistence with the scenario lined by relaxation data and further supported by diffusion measurements reported elsewhere. [52] Overall, in deacidified samples, the unilateral NMR measurements show a restructuring of cellulose texture in ADs even after an acidification bath and a subsequent ageing under strong conditions. This recovery can be associated to the action of alkaline nanoparticles used for the deacidification treatment, thus proving that this protocol effectively hampers, from the point of view of cellulose arrangement, the degradation triggered by artificial ageing.…”

“…The final effect of ageing causes A1 sample to have the same confining size of R1, whereas that of the A2 sample is reduced with respect to R2; this is fully confirmed by diffusion data reported elsewhere. [52] This means that water mobility is similar in A1 and R1 samples, whereas it is reduced in A2 with respect to R2 sample. These conclusions are fully supported by confining sizes obtained by diffraction dips (Figure 4), even though they scale with different relative amplitudes, as T2 monitors the confining size by means of the local magnetic field.…”

“…Further support can be found in NMR diffusometry data, reported elsewhere. [52] The relaxation maps of acidified samples are reported in Figure 3. The acidification bath used for the preparation of these samples (see Materials and Methods section) induced significant decrease in cellulose DP (see Table 1) without triggering crosslinking if compared with R samples' artificial ageing at high temperature and relative humidity.…”

Advanced Materials in Cultural Heritage Conservation

“…In particular, colloidal earth-alkaline hydroxides and carbonates in organic solvents have been employed to adjust the pH of cellulose- and collagen-based artifacts (paper, wood, parchment, and leather) [ 78 , 79 , 80 , 81 ]. The beneficial effect of alkaline nanoparticles for the preservation of acidic paper has been also recently shown by NMR diffusometry and relaxometry measurements [ 82 , 83 ].…”